1. Field of the Invention
This invention relates to the prevention of surface precure of thermosetting amine-aldehyde resins in the manufacture of composition board.
2. Description of the Prior Art
"Surface precure," as used herein refers to premature instantaneous curing of the thermosetting resin at the composition board surface, which results in the formation of a thin surface layer of "case hardened" material. Precure is caused by heat transfer from the high temperature presses normally employed during consolidation of the board. The case hardened surface layer prevents uniform consolidation and curing of the board by interfering with proper heat transfer into the center of the board. In addition to structural inferiority occasioned by nonuniform curing, the prior art board surface possesses a poor appearance and feel which can only be removed by deep sanding. Thickness and material loss upon sanding, however, decrease the strength characteristics of the board even further.
Prior art attempts to control surface precure in composition board applications have centered around the use of urea as a cure retarding agent. See, e.g., Menger, U.S. Pat. No. 2,236,184 and Peters, U.S. Pat. No. 3,649,307. Urea commonly is added to urea-formaldehyde resins either during the resin forming step or to the finished resin as a post-additive. Urea addition, however, has not provided suitable precure prevention. Urea exhibits negligible precure resisting effects at the high temperature conditions which generally give rise to precure in composition board processes. Moreover, the increased urea ratio in such an extended resin results in excessive penetration and flow which adversely affect the board strength.